The present invention relates to a flexographic machine.
It is known that in conventional flexographic machines there are printing units formed by a first cylinder, which supports a printing plate designed to apply ink to the material to be printed, by a central drum, on which the material to be printed rests, by an anilox roller, which is responsible for inking the printing plate, and by an inking unit, which in turn is designed to wet the anilox roller with ink.
The surface of the anilox roller is provided with etched microcells (hence the term anilox), which are designed to be filled uniformly by the inking unit.
Generally speaking, the inking unit has a doctor body, which is supported laterally by sliders that can move on guides supported by the two lateral shoulders of the flexographic machine.
Two doctor blades are mounted on the doctor body and are placed in contact with the surface of the anilox roller during its inking, in order to remove the excess ink and achieve uniform distribution of the ink on the anilox roller.
In particular, the doctor body delimits, together with the doctor blades, two lateral sealing gaskets and the sector of the anilox roller that is affected by the doctor blades, an inking chamber into which the ink is sent at a pressure that depends on its viscosity.
The frequent operations for cleaning the doctor body and replacing worn blades generally require removing and subsequently assembling the doctor body with respect to the flexographic machine.
Since these operations are performed manually by the operator, the weight of the doctor body must be limited.
Accordingly, such body is usually made of lightweight material, such as aluminum or other light alloys.
However, its limited weight limits the flexural rigidity of the doctor body, although such rigidity is decisive for the correct inking of the anilox roller.
In order to achieve correct inking of the anilox roller, it is critical that the contact pressure between the doctor blades and the surface of the anilox roller, known as doctor pressure, be constant over time and uniform over the entire length of said anilox roller.
For this reason, the inking unit is provided with devices, generally constituted by linear actuators that usually act by means of the lateral supports of the doctor body, which are designed to apply and control doctor pressure.
The pressure of the ink inside the inking chamber and the force applied to the doctor body in order to ensure that contact is maintained between the doctor blades and the anilox roller tend to generate an inflection of the doctor body due to its reduced flexural rigidity.
Such inflection causes a separation of the central portion of the doctor blades from the surface of the anilox roller or in any case a reduction of the doctor pressure, with a consequent degradation of the inking conditions and the consequent onset of uneven wear of the doctor blades.
The extent of the inflection affecting the doctor body during inking of the anilox roller of course also depends on its length and therefore on the overall width of the flexographic machine.
In order to try to solve the problems linked to the inflection of the doctor body, inking units have already been devised which are provided with a doctor body having stiffening ridges designed to increase its flexural rigidity or is optionally provided with a resisting cross-section that has a high moment of inertia.
These technical solutions have not proved to be fully adequate, since the possibility to act on the flexural rigidity of the doctor body is greatly limited by its above mentioned low weight requirements.
Another proposed solution uses a doctor body being supported by a beam that has, in its central part, a protrusion that is designed to bend the doctor body in the inactive condition in order to compensate for the inflection that affects it in operating conditions.
Although this embodiment is valid from a conceptual standpoint, it has proved to be scarcely practical both from the functional and from the production standpoints.
First of all, it is in fact capable of ensuring a sufficient compensation of the deformation affecting the doctor body only in very specific operating conditions and not in all conditions.
Secondly, the beam, in addition to being complicated to manufacture, is particularly bulky, and therefore limits the possibility to clean the anilox roller, makes it awkward to remove the ink collection tray located below the inking unit, and finally forces to locate the points where the ink enters and exits from the inking chamber in specific positions that are not affected by the beam and by the system for fixing said beam to the doctor body.
The aim of the present invention is to provide a flexographic machine that is capable of providing a valid solution to the technical problem cited above and is capable of eliminating the drawbacks noted above.
Within this aim, an object of the present invention is to provide a flexographic machine that has a doctor body provided with such a flexural rigidity that it can withstand adequately the flexural stresses that affect it in any operating condition, yet at the same time has a limited weight so as to allow its manual handling by operators.
Another object of the invention is to provide a flexographic machine that allows to replace the doctor blades rapidly and without the aid of tools for the operator.
A further object is to provide a flexographic machine that has an inking unit whose structure is very simple, highly durable, simple to manage and competitive in terms of manufacturing cost.
This aim and these and other objects that will become better apparent hereinafter are achieved by a flexographic machine according to the invention, which comprises at least one inking unit that has lateral supporting means for a doctor body that forms at least partially an inking chamber and supports at least one first doctor blade and at least one second doctor blade, said at least one inking unit comprising means for the advancement/retraction of said doctor body that are adapted to move it from a retracted position to a forward position, in which said at least one first doctor blade and said at least one second doctor blade engage an anilox roller in order to ink said anilox roller, said at least one inking unit further comprising means for feeding the ink to said inking chamber and means that contrast the inflection of said doctor body that occurs when said inking unit is in the active condition, and is characterized in that said means that contrast the inflection of said doctor body comprise means that are adapted to apply a contrasting flexural action to said doctor body, at least when said inking unit is in the active condition, said contrasting flexural action being substantially equal and opposite to the flexural action induced on said doctor body when said inking unit is in the active condition.
Advantageously, such means adapted to apply a contrasting flexural action comprise bending means that are adapted to bend said doctor body and means for adjusting the bending of said doctor body.